Open your finished 2560x512 image. Make sure that your background layer is transparent. Up top click on Layer and then choose Merge Visible.

Choose your Slice tool (or hit K on your keyboard). Right click over your image and choose Promote to User Slice. Right click once more over your image and choose Divide Slice. Put a tick in the box for Divide Vertically Into and you want 5 slices across, evenly spaced. (Put a tick in the box for Preview if you want, although not needed.) Click Ok.

Go up to File and choose Save for Web. This will open an Image Ready window. On the right side of that window from the Preset dropdown menu choose PNG-24 and put a tick in the box for Transparency. On the left hand side, be sure that the Slice Select Tool is chosen. Click Save. (This assumes you've already created an empty folder to place the images.) Click Save once more and it will save all 5 sliced images into a subfolder named "images".

You will need the image converter located here on WoWI. Drop the exe file into the folder where your sliced images are located. Now (with your mouse) drag one png file at a time over the top of the BLP2PNG.exe file and it will convert it to a blp file. Do that with your remaining png files and you're done converting your images

This is the tool called "WoWImage" I use to convert from PNG to BLP and visa versa... it's a really nice, easy to use, applet. You can drag and drop multiple files onto it at one time to batch convert them, as well, as opposed to one file at a time.